Method of electrolytically coating a metallic object with platinum



United States Patent METHOD OF ELECTROLYTICALLY COATING A METALLIC OBJECT WITH PLATINUM Popko Reinder Dijksterhuis and Albertus Remko Hovingh, Einrihoven, Netherlands, assignors, by mesne assignments, to North American Philips Company, inc, New York, N. Y., a corporation of Delaware No Drawing. Application September 18, 1956 Serial No. 610,675

Claims priority, application Netherlands October 19, 1955 3 Claims. (Cl. 204-37) My invention relates to methods of electrolytically coating metallic objects, particularly molybdenum, with platinum.

Heretofore, molybdenum has been plated with platinum using electrolytic baths containing in addition to a platinum salt, another metal salt, such as a lead salt, capable of increasing the platinum deposit from the bath. In this method some of the metal other than platinum is plated out together with the platinum and cannot be separated from the platinum in the coating. This is highly disadvantageous for certain purposes namely where the molybdenum object is to serve as an electrode of an electric discharge tube.

A principal object of my invention therefore, is to develop a process for electrolytically coating metallic objects with platinum which eliminates the presence of any undesired metallic contaminate in the finally sintered platinum layer.

A second principal object of my invention is to increase the platinum output of the electrolytic bath used in coating metallic objects with platinum.

Still another object of my invention is to increase the thickness of the platinum layer deposited by the electrolytic process.

These and further objects of my invention will be apparent from the following description.

According to my invention I have unexpectedly found that by electrolytically depositing the platinum from an aqueous solution of chloroplatinic acid (H PtCl and mercuric chloride (HgCI the platinum output of the bath is greatly increased and the mercury that is deposited simultaneously with the platinum black is almost completely eliminated during the sintering operation. Due to the ease of elimination of the mercury deposit from the platinum layer I have found that by the process of my invention I can rapidly build up platinum layers of almost any desired thickness without risk of contamination with mercury or any other metal.

While almost any concentration of chloroplatinic acid and mercuric chloride is suitable for the purposes of my invention I have found that better results are obtained if the concentration of these compounds is from 150 to 300 gms. of chloroplatinic acid and from 1 to 15 gms. of mercuric chloride per liter of solution. I have found that optimum results are obtained if the electrolytic bath contains about 240 gms. of chloroplatinic acid and 7 gms. of mercuric chloride per liter.

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During the electrolytic coating step I prefer to utilize a relatively high current density i: e. 50-150 amps./cm.

Further I prefer to form a thick layer by the sintering of several thin layers. Especially in the case when the object to be coated is molybdenum I have found that it is important for the first layer of platinum to be very thin i. e. from about 0.5 to 0.7 in order to avoid cracking of the platinum layer after the sintering step. In order to build up a thick coating of platinum, this initial thin layer may then be coated by subsequent thick layers.

My invention will now be more fully described with reference to the following example:

Example A molybdenum wire having a thickness of 175 .0 was passed through an electrolytic bath 30 cms. in length consisting of an aqueous solution containing 240 gms. of chloroplatinic acid and 7 gms. of mercuric chloride per liter, at a speed of meters per hour. During this operation a current of 1.5 amps. was passed through the solution, the molybdenum wire being connected to the cathode.

After sintering at 1000 C. the wire was found to have a coating of substantially pure platinum from 0.5 to 0.7 thick. By repeating this operation a number of times it was found possible to build up substantially pure, dense platinum layers up to 10p. thick.

While I have described my invention in connection with specific embodiments and applications, other modifications thereof will be readily apparent to those skilled in this art without departing from the spirit and scope of the invention as defined in the appended claim-s.

What is claimed is:

1. A method of electrolytically coating a metallic object with platinum comprising the steps, immersing the metallic object in an electrolytic bath comprising an aqueous solution of chloroplatinic acid (H PtCl and mercuric chloride, the concentration of the chloroplatinic acid being from about to 300 gms. per liter and the concentration of the mercuric chloride being about 1 to 15 gms. per liter, while passing a suitable current through the bath for a time sufficient to deposit a layer of platinum black of the desired thickness, and sintering said layer at a temperature of about 1000 C. to convert it to platinum substantially free of mercury.

2. The method of claim 1 in which the current density ranges from about 50 to 100 amps/cm. and the bath contains 240 gms. of chloroplatinic acid and 7 gms. of mercuric chloride per liter.

3. The method of claim 1 in which the metallic object has a surface comprising molybdenum.

References Cited in the file of this patent UNITED STATES PATENTS OTHER REFERENCES Beneker May 18, 1909 

1. A METHOD OF ELECTROLYTICALLY COATING A METALLIC OBJECT WITH PLATINUM COMPRISING THE STEPS, IMMERSING THE METALLIC OBJECT IN AN ELECTROLYTIC BATH COMPRISING AN AQUEOUS SOLUTION OF CHLOROPLATINIC ACID (H2PTCL6) AND MERCURIC CHLORIDE, THE CONCENTRATION OF THE CHLOROPLATINIC ACID BEING FROM ABOUT 150 TO 300 GMS. PER LITER AND THE CONCENTRATION OF THE MERCURIC CHLORIDE BEING ABOUT 1 TO 15 GMS. PER LITER, WHILE PASSING A SUITABLE CURRENT THROUGH THE BATH FOR A TIME SUFFICIENT TO DEPOSIT A LAYER OF PLATINUM BLACK OF THE DESIRED THICKNESS, AND SINTERING SAID LAYER AT A TEMPERATURE OF ABOUT 1000*C. TO CONVERT IT TO PLATINUM SUBSTANTIALLY FREE OF MERCURY. 